Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
As critical dimensions shrink, better lithographic resolution is needed to print smaller features. Lithography depth of focus is being reduced, which means that minor problems will affect lithography focus, such as alignment issues or mechanical vibration. In lithography, an out-of-focus exposure of features in a photoresist can smear the photoresist edges, which can result in incorrect lateral feature dimensions. The semiconductor yield will suffer if this defocus is not detected. Without effective inspection, defocus problems may not be detected until many manufacturing steps after the lithography step.
Existing macro-inspection systems will find it more difficult to detect defocus as critical dimensions shrink. The development of new techniques with increased sensitivity for defocus detection, both for localized and extended defocus defects, will be important as critical dimensions continue to decrease.
Conventional algorithms perform defect detection on data from each color source independently and then combine the detection result. No single color source is sensitive to all defocus values. The overall defocus detection is poor with only strong defocus being detectable. Therefore, an improved defocus detection technique is needed.